Also keep in mind that if your rear seat has a "pass through", it needs to be locked as well.
My Accord has multiples entry methods into the trunk, including:
Traditional key in the lid
Lever in the interior
Remote when it works (25% of the time)
Pull down the rear armrest and there is a lockable pass-through
Above this pass-through is a lever to pull from inside the trunk if you want to fold the seat down
And a keyhole in the rear deck
Now, let's say I locked out the trunk lever inside but didn't lock the pass-through. I could stick my arm in it and pull the seat down. Make sure every measure is locked up if things are going to be in the trunk.
Lucky (or maybe unlucky?) me...I could always file an insurance claim if my stereo stuff was stolen since it's worth more than the deductible.
Thieves are really the lowest form of life...
(Edit: come to think of it...someone could just remove my rear speaker grilles and reach through to the lever and fold the seat down since I don't have rear speakers. Lol you didn't hear that
)